To call Sydney Kentridge the outstanding barrister of his generation is not a huge claim, since few are still working at 90 years old. Sir Sydney, who started his career as a lawyer in South Africa in 1949, marked his 90th birthday in November by representing the Law Society in a constitutional case in front of the UK Supreme Court.

The fact that he is still on his feet after six decades is, however, only one reason for his eminence. He has represented three Nobel Prize winners in court – Nelson Mandela, Desmond Tutu and Chief Albert Luthuli. At the inquest in 1977 for Steve Biko, the South African anti-apartheid activist who was beaten to death by police, he appeared for Biko’s family.

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Lunch with the FT

He has also had an amazingly diverse career. Unlike other lawyers, who specialise in aspects of commercial or criminal law, he has done nearly everything, taking cases of murder, libel, corporate takeovers, patent protection, constitutional law and much else on two continents.

He has done it all in a quiet, determined way, the opposite of the flamboyant courtroom attorney. “His manner was always understated, controlled and relentlessly rational. His cross-examination was devastating. Of the private person, there were few glimpses,” Mandela once said of Kentridge, who represented him in the Treason Trial in the late 1950s.

I have known Kentridge slightly for some years, as a friend of his children, but was astonished to review his career in preparation for our lunch. When he arrives at Bentley’s Oyster Bar & Grill in Piccadilly, a restaurant where he first ate in the late 1960s, he is attired as you might expect a distinguished lawyer to be: three-piece pinstriped suit, a shirt with a white collar and cufflinks from his Oxford college. He looks a decade or two younger than his age.

Kentridge has wiry white eyebrows and a gentle, penetrating expression. He has a self-contained, sturdy quality – like the South African rhinoceros that recurs in the work of his son William, a renowned artist and opera director. The restaurant he has chosen, taken over by the chef Richard Corrigan eight years ago, is a far cry from his first experience of British food, when he arrived in England to study at Oxford in 1946.

“There was still rationing and the food was very poor,” he says. “There used to be something called British Restaurants and you could get a three-course lunch there for one and threepence. If you had bread, that was one course, then there was usually some watery soup and then a bit of meat, not very much, with some potatoes and sprouts or broccoli.”

Even bombed-out London was exciting for a 24-year-old, demobbed from the South African army, who’d been born into a liberal Jewish family in Johannesburg. “We very much thought of ourselves as part of the British empire ... All the books we read were English – Billy Bunter, the public school stories. We had a Monopoly board with the streets of London so we knew that Old Kent Road was downmarket and Mayfair was expensive.”

The Oyster Bar is subtly restored from its 1916 origins, with scalloped wall-lamps and red banquettes. Diners line the bar and the small tables are squeezed close together: it is amiably noisy. Kentridge orders herrings with beetroot and mustard to start and then sole goujons with duck egg mayonnaise. I choose tuna with soy and mirin, followed by fish pie. He selects a glass of Riesling for each of us.

. . .

As we meet, Mandela has just been taken to hospital with a lung infection and gall bladder problems, causing concern that the father of the modern South Africa is about to die. It sounds serious, I remark. “At that age, I think anything is serious,” he replies soberly, somehow making it sound as if he is of a different generation to his 94-year-former client.

I ask if they first met at the Treason Trial, a three-year mass examination of the African National Congress leaders in Pretoria. “I might have met him before the trial. He and Oliver Tambo had a firm of attorneys called Mandela and Tambo. I got two or three briefs from them to defend in small criminal cases in the magistrates’ court.” Given the course of history, it sounds curiously mundane.

At Mandela’s trial, he says, “You could see his quality of leadership from the beginning ... He was very thoughtful. I spent quite a lot of time talking to him and, unlike some of the others, he never went in for slogans. If you asked him a question about policy or their activities, he’d always think about it, he wouldn’t give a flip answer ... He was calm about it. He never took things personally.” It strikes me that Kentridge’s description of Mandela matches Mandela’s of Kentridge. They must have made a formidable combination in court, and Mandela was acquitted, along with the other main defendants. Mandela was later jailed, in 1962, having founded the ANC’s armed wing, and only freed in 1990.

Our food comes promptly and Kentridge munches his way through his herring – age has clearly not withered his appetite. We discuss how he chose his career. He’d been tempted by journalism – “I think I could have become a good leader writer and also a good investigative journalist” – but his mind was changed by participating in some army courts-martial. “I thought it was the most interesting thing I’d ever done.”

The postwar South African bar was modelled on the London law courts but it could be a struggle to win cases in front of politically appointed judges in a country with few liberties for non-whites. Kentridge recalls the warning he received from a fellow defence lawyer.

“He said, ‘When you’re doing one of these [ANC] terrorism cases, you can be certain of three things. First, the witnesses have all been assaulted, or threatened with assault, by the police. Secondly, any statement made to the police by the accused has been obtained by torture. And thirdly, your client is guilty.’ ” Kentridge leans back to chuckle at the anecdote.

At the Biko inquest, after Kentridge had destroyed the police evidence on the stand, and described the activist’s “lonely and miserable death”, the magistrate found that no individual was to blame. Wasn’t it demoralising to work in a country where the odds were so stacked against you?

He ponders it. “The answer was, you had to do what you could.”

“You never considered it pointless?

“It might have been true, but it was irrelevant. The black activist’s view was that he wanted a lawyer.”

He recalls how one armed group of ANC members, caught trying to blow up a police station, refused to recognise the court or be represented by lawyers. They disrupted the trial by singing songs. Most were given 10-year sentences but one, who had been particularly troublesome in court, got a death sentence. Kentridge was summoned to save him.

“So I was the advocate, and what happened was, I read the whole proceedings of the court,” he says, eyes gleaming at the memory. “I saw that this chap wasn’t the ringleader of the plot, he was just the ringleader of the contempt of court. We went to appeal and the judges saw it the same way I did. They set aside the death sentence and I think he got 10 years.”

We move to our main courses – mine a thick, creamy fish pie with huge prawns and chunks of salmon. Kentridge proceeds staunchly with a platter of golden fish and a mound of mayonnaise. Sometimes, after starting a reply, he pauses for so long to eat that I wonder whether he has ceased talking but his answers eventually resume where he left off.

I ask his view of today’s ANC government under Jacob Zuma. “I think they’re very disappointing,” he says firmly. “There are still hundreds of thousands of people, millions perhaps, who are homeless or living in slums, and the government doesn’t have a sense of urgency. There’s widespread corruption but it does not seem to be able to do anything.”

It was his wife who encouraged him to move professionally to London in the late 1970s. “Felicia had a theory that everyone should change his occupation at least every 25 years. I’d got quite tired of doing the same cases at the bar in South Africa over and over and over. In another dispensation, I’d have become a judge but that obviously wasn’t going to happen.”

Even at Lincoln’s Inn Fields, he resisted the pressure to specialise? “Well, first of all, I don’t know that I knew enough about any subject to be a specialist. And, secondly, I knew some very good ones and I thought it would be terribly dull to do one thing. I met a man who did patents. But he didn’t do all patents. He only did pharmaceutical patents. He knew more about those than anyone. I thought to myself, ‘What a life!’ ”

. . .

Despite this determination not to be pinned down, Kentridge not only became a Queen’s Counsel and was knighted, but turned into one of the UK’s most valued constitutional lawyers. Why, though, does he still practise?

“There is no retiring age at the bar and I’ve been in the fortunate position of being able to take cases of interest to me,” he says. “If I’d been a judge, I’d have had to retire 20 years ago.”

Nobody knocks on your door and says, “That’s it”?

“They can give you a hint, which I’ve now taken. I think I’ll retire some time [this] year. You can’t go on for ever.”

As I digest this announcement, the waiter comes to remove our empty plates. Kentridge gives his order with James Bond-like precision. “I’d like an ordinary filter coffee, please – an Americano, I think you call it – with milk on the side. Not boiled, warm.” He makes it sound so appealing that I order the same before asking him about his son William. It is highly unusual for two family members to achieve such eminence in such widely varying fields, I remark.

“I suppose so,” he says doubtfully. “Did you ever come across Ben Hytner? He’s a very distinguished barrister and his son is Nicholas Hytner [director of the National Theatre]. I’ll tell you a story. You know a writer and critic called Adam Mars-Jones? His father was a high court judge who suffered from what the bar calls ‘judge-itis’ – he was very conscious of his position. The time came when he retired and Ben Hytner gave the tribute on behalf of the bar. He said, ‘There is one distinction that your Lordship and I share. We both have sons who are more distinguished than we are.’ He infuriated the judge and delighted the bar. I told William that story.” He beams.

We sip our coffee and I press him to explain why he values the rule of law so highly. The question seems to take him aback, as if it is either too obvious or too tricky to answer. “I gave an address on the subject at the Commonwealth Law conference in Hyderabad [in 2011]. If you like, I could send you a copy,” he offers.

How about a précis? I retort. Kentridge regards me wearily, as if I am demanding too much for a lunch. Then he starts a lengthy, intricate disquisition on the roots of constitutional law. He concludes with a story about the arrest of a Russian ambassador in the reign of Queen Anne, which so upset the Tsar of Russia that he wrote, demanding that those responsible be executed.

“Queen Anne sent a wonderful reply, saying the Tsar must understand that she has no power to put even the lowliest citizen to death, save by the operation of the law, and she trusts that the Tsar will not compel her to impossibilities.” He places an elbow on the table and slowly wags a long forefinger at me in epiphany. “That’s the rule of law.”

So, after all these years, with such respect for the UK’s unwritten constitution, does Sir Sydney feel British?

“Well, it’s very hard to say,” he muses. “At cricket, I want South Africa to win. In other sports, I support England. When England plays rugby against South Africa, I back England. It used to be very much the nationalist game and they have the odd black player but it’s the same old people who run it. So, as far as they go, no.”